s3: Modifications to generic notify structures to allow implementation of OneFS notify.

s3: Modifications to generic notify structures to allow implementation of OneFS notify.

The OneFS kernel based change notify system takes an fd of the directory
to watch in it's initialization syscall.  Since we already have this
directory open, this commit plumbs that fd down to the VFS layer via the
notify_entry struct.

We also need to know if the watch is taken out on a snapshot directory.
The full file_id struct is also passed down to make this determination.
The file_id marshalling wrappers are hand written here, but should
eventually be auto-generated by moving the struct file_id into the idl.